In Spain is not easy for the merchant if they want to do it legally... From a fiscal advisor (José Antonio Bravo):

"This is what a business needs to do to accept payments in BTC. You can create the invoice in Euros (which is what the tax authorities care about) and include the conversion to the equivalent in BTC. However, you must consider that this will be the acquisition price of the BTC so that, when you sell them, you can calculate whether there has been a gain or loss.

In addition to the above, you must account for the BTC in an accounting entry in accordance with its future destination.

If you are planning to convert them into euros within a period of less than a year, you should record the entries in an Inventory account (since they are not money, nor financial assets), and maintain this inventory account by the speculative method. That is, the inventory enters at the purchase price and exits at the selling price at the time of its conversion to euros, or its exchange for other goods or services (including exchanges for other cryptocurrencies). At the end of the year, a regularization of inventory is performed based on the remaining units at that time at their value (this can be done, for example, through Cointracking), and the difference, positive or negative, is taken to results.

However, if you intend to hold the cryptocurrencies as an investment, you should record the entry as Fixed Assets, with the particular characteristic that, being assets without depreciation over time (as could happen with land), their value cannot be amortized. It would be advisable to detail them by batches of different prices. When it is decided to sell these assets in exchange for money or other goods or services, it would be convenient to change their categorization to Inventory."